Proportional pressure reducing valve

ABSTRACT

A proportional pressure reducing valve includes a spool having a first end connected to a solenoid and a second end exposed to a feedback chamber. The spool is formed with a groove for allowing a supply port to communicate with an output port when the spool is driven by the solenoid in proportion to solenoid current under balanced condition of an axial force of the solenoid and feedback pressure. The pressure reducing valve further comprises a pressure reduction valve including a second spool formed with a path, for relieving hydraulic pressure within the feedback chamber to the drain port, when output port pressure rises a predetermined relief pressure, to sharply increase the output port pressure up to the supply pressure in proportion to the current passed through the solenoid.

RELATED APPLICATION

This application is a division of our prior copending application for U.S. Pat. entitled "PROPORTIONAL PRESSURE REDUCING VALVE" which was filedOct. 7, 1988 and which bears Ser. No. 07/254,896 now U.S. Pat. No.4,899,785.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a proportional pressure reducing valve,and more specifically to a direct acting proportional pressure reducingvalve used for controlling a clutch assembly incorporated in anautomatic transmission for an automotive vehicle, for instance.

2. Description of the Prior Art

An example of proportional pressure reducing valves is disclosed inJapanese Published Unexamined (Kokai) Utility Model Application No.60-142371. In this prior-art proportional pressure reducing valve, aproportional solenoid composed of a coil, a plunger, and a push rod isprovided at an end of a spool slidably disposed within a central holeformed in a valve housing, and an output port pressure of the valve iscontrolled by the spool under balanced condition of an axial forcegenerated by the solenoid and applied to one end of the spool and anoutput port pressure introduced into a feedback chamber and applied tothe other end of the spool. This output port pressure is introduced intoa clutch assembly for an automatic transmission for an automotivevehicle to control clutch engagement operation.

When the above-mentioned valve is used to control a clutch assembly of aautomatic transmission, a high control precision and a high pressureresponse speed are both required for the valve to improve transmission(speed change) feeling or reduce transmission (speed change) shock.

In more detail, the precision of the clutch pressure control operationis closely related to transmission shock. That is, to reducetransmission shock, it is necessary to first keep the clutch under halfengagement condition and then under perfect engagement condition bycontrolling the hydraulic pressure, after a sufficient rotary force hasbeen transmitted. In order words, in case the precision of the clutchpressure control is not high, the half clutch engagement is notobtained, so that a great transmission shock is generated.

On the other hand, the clutch response speed is determined by a delaytime from when a clutch assembly piston begins to move in response to anoutput pressure of the proportional pressure reducing valve to whenclutch disks are engaged with each other to transmit a rotary force.Therefore, it is important to reduce the above-mentioned delay orwasteful time. This delay time is dependent upon pressure loss in thevalve and piping system. Therefore, there inevitably exists a lowerlimit of the opening area of the spool valve in order to reduce pressureloss of the valve.

In the case of a cylindrical port, the opening area S can be expressedas

    S=π·D·l                               (1)

where D denotes the spool diameter, and l denotes a spool openinglength. Further, an axial force F required for the proportional solenoidcan be expressed as

    F=1/4·πD.sup.2 ·P                     (2)

where P denotes output port pressure of the pressure reducing valve.

Therefore, in order to reduce the delay time and to improve the responsespeed, the opening area must be increased, and therefore it is necessaryto increase the opening length l or the spool diameter D. However, whenthe opening length l is increased, it is necessary to increase theeffective stroke of the proportional solenoid, so that there exists aproblem in that the solenoid dimensions increase.

On the other hand, when the spool diameter is increased, it is necessaryto increase the axial force F of the proportional solenoid, so thatthere exist other problems in that the solenoid dimensions increase andfurther the solenoid current consumption rate increases. On the otherhand, when the proportional solenoid dimensions are restricted, thecontrollable pressure range may be narrowed.

In summary, in the prior-art valve, the controllable pressure range isinevitably narrowed, when the size and the current consumption of theproportional solenoid are reduced under consideration of mounting spaceand heat generation, thus resulting in a problem in that the clutcheasily slides when the clutch pressure is low and the clutch torque ishigh. Therefore, the response speed and the controllable pressure rangeare determined by finding an appropriate point of compromise.

To overcome the above-mentioned problem, other structures such that atwo-step spool having the feedback portion whose diameter is smallerthan that of the port portion or valves of pilot type have beenproposed. In the case of the two-step spool, however, the concentricityof the two different diameter portions must be precise and therefore themanufacturing cost is high. Further, since the spool driving force issmall, there exists another problem in that the valve is subjected toexternal disturbance such as friction or hydraulic pressure.

On the other hand, in the case of the pilot valve, the structure iscomplicated; the manufacturing cost is high; the size is large, so thatthere are many factors related to delay time and therefore the responsespeed is low.

SUMMARY OF THE INVENTION

With these problems in mind, therefore, it is the primary object of thepresent invention to provide a proportional pressure reducing valvewhich is high in control precision and response speed and large incontrollable pressure range without increasing the size and the currentconsumption of the proportional solenoid.

To achieve the above-mentioned object, a proportional pressure reducingvalve according to the present invention comprises: (a) a valve housingformed with a supply port, an output port, a drain port, and a firstfeedback chamber communicating with the output port; (b) a proportionalsolenoid attached to said valve housing; (c) a spool having a first endconnected to said solenoid and a second end exposed to the feedbackchamber, said spool formed with a groove for allowing the supply port tocommunicate with the output port, when said spool is driven by saidproportional solenoid in such a way that output pressure at the outputport increases in proportion to current passed through said solenoidunder balanced conditions of an axial force applied to the first end ofsaid spool by said proportional solenoid and a feedback force applied tothe second end of said spool within the feedback chamber; and (d) apressure reduction valve including a second spool formed with a path,for relieving hydraulic pressure within the feedback chamber to thedrain port, when output port pressure rises beyond a predeterminedrelief pressure, to sharply increase the output port pressure up to thesupply pressure in proportion to the current passed through thesolenoid.

The above-mentioned pressure reduction valve is disclosed in FIG. 4A asthe fourth embodiment.

Where the valve of the present invention is applied to control a clutchassembly for an automatic transmission, if the solenoid current is equalto or lower than a predetermined relief current, since hydraulic outputpressure increases in proportion to the solenoid current, it is possibleto gradually engage the clutch under half clutch engagement conditions.On the other hand after an sufficient rotary force has been transmitted,if the solenoid current increases more than the relief current, sincethe hydraulic output pressure increases sharply up to the supplypressure, it is possible to completely engage the clutch under stableclutch engagement conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a cross-sectional view showing a first embodiment of theproportional pressure reducing valve of the present invention;

FIG. 1(B) is a graphical representation showing solenoid current-outputpressure characteristics of the first embodiment shown in FIG. 1(A);

FIG. 1(C) is a similar graphical representation showing other solenoidcurrent-output pressure characteristics of the first embodiment, inwhich a preset relief pressure is changed;

FIG. 2(A) is a cross-sectional view showing a second embodiment of theproportional pressure reducing valve of the present invention;

FIG. 2(B) is a graphical representation showing solenoid current-outputpressure characteristics of the second embodiment shown in FIG. 2(A);

FIG. 2(C) is a cross-sectional view showing another modification of thesecond embodiment shown in FIG. 2(A);

FIG. 3(A) is a cross-sectional view showing a third embodiment of theproportional pressure reducing valve of the present invention;

FIG. 3(B) is a graphical representation showing solenoid current-outputpressure characteristics of the third embodiment shown in FIG. 3(A);

FIG. 3(C) is a similar graphical representation showing other solenoidcurrent-output pressure characteristics of the third embodiment;

FIG. 4(A) is a cross-sectional view showing a fourth embodiment of theproportional pressure reducing valve of the present invention;

FIG. 4(B) is a graphical representation showing solenoid current-outputpressure characteristics of the fourth embodiment shown in FIG. 4(A);

FIG. 5(A) is a cross-sectional view showing a fifth embodiment of theproportional pressure reducing valve of the present invention;

FIG. 5(B) is a graphical representation showing solenoid current-outputpressure characteristics of the fifth embodiment shown in FIG. 5(A);

FIG. 5(C) is a similar graphical representation showing other solenoidcurrent-output pressure characteristics of the fifth embodiment, inwhich spring load is set lower; and

FIG. 5(D) is a cross-sectional view showing another modification of thefifth embodiment shown in FIG. 5(A);

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the proportional pressure reducing valve of thepresent invention will be described with reference to FIG. 1(A), (B) and(C). In FIG. 1(A), the valve 1 is roughly composed of a proportionalsolenoid section 2 and a valve section 3. The proportional solenoidportion 2 includes a coil 4, a plunger 5, and a push rod 6.

The valve section 3 includes a valve housing 8 formed with an insertionthrough hole 9 at the center thereof. This insertion through hole 9communicates with a supply port 10 connected to a hydraulic pressuresource 13, a drain port 11 connected to a tank 14, and an output port 12connected to a clutch assembly 16 via a pipe 15.

A spool 17 is slidably disposed within the insertion through hole 9. Oneend surface of the spool 17 (on the right side) is in contact with apush rod 6 of the solenoid section 2. The spool 17 is formed with afirst land portion 18 facing the supply port 10, a second land portion19 facing the drain port 11, and a groove portion 20 facing the outputport 12 formed between the two ports 10 and 11.

The other end of the spool 17 (on the left side) communicates with afeedback chamber 23 communicating with the output port 12 via a feedbackpath 21 and a damping throttle 22. Further, the spool 17 is urged towardthe solenoid section 2 by a return spring 24 disposed within thisfeedback chamber 23. Further, the feedback chamber 23 communicates withthe drain port 11 via a throttle 25 and a path 26. A relief valve 29composed of a spring 27 and a ball 28 is provided at this throttle 25.

The clutch assembly 16 includes a first rotary body 31, and a secondrotary body 32 rotatable relative to the first rotary body 31. Firstclutch discs 33 are slidably spline fitted to the first rotary body 31so as to be rotatable therewith, and a second clutch disc 34 is alsoslidably spline-fitted to the second rotary body 32 so as to berotatable therewith. These two clutch discs 33 and 34 are alternatelydisposed. A piston 35 is slidably disposed within the first rotary body31, and urged by a return spring 36. Further, the rotary body 31 isformed with a stopper 37.

The operation of the first embodiment thereof will be describedhereinbelow.

When current flows through the coil 4 of the proportional solenoidsection 2, an attraction force proportional to the current is producedat the plunger 5 to urge the spool 17 via the push rod 6 in the leftwarddirection. Therefore, the supply port 10 communicates with the outputport 12 via the groove portion 20, so that output pressure at the outputport 12 rises. The pressure at this output port 12 is introduced intothe feedback chamber 23 via the feedback path 21 and therefore pressurewithin the feedback chamber 23 rises to move the spool in the rightwarddirection. That is, the spool 7 is balanced at a position where an axialforce generated by the solenoid and applied to the rightward end of thespool 7 and a force obtained by the pressure within the feedback chamber23 and applied to the left end of the spool 7.

FIG. 1(B) shows the relationship between the solenoid current I and thevalve output pressure P. In FIG. 1(B), the output pressure P increasesin proportion to an increase in the solenoid current I. However, whenthe current reaches a relief current I_(r), the pressure within thefeedback chamber 23 reaches a relief pressure P_(r) determined by thespring 27 of the relief valve 29 at the throttle 25, so that thepressure P_(r) within the feedback chamber 23 is released into the tank14 via the drain port 11. In other words, when the solenoid currentincreases beyond this predetermined relief current valve I_(r), sincethe pressure within the feedback chamber 23 is released, the spool isnot balanced, because the axial force of the solenoid exceeds thepressure force within the feedback chamber, so that the spool 7 is movedin the leftward direction. Therefore, the supply port 10 is full openedand therefore the output pressure P reaches a supply pressure P_(s) ofthe hydraulic pressure source 13. In the above operation, the elasticforce of the return spring 24 is weak enough to full close the supplyport 10 and the drain port 11 when no current is supplied to the coil 4,this return spring 24 will not exert a harmful influence upon the aboveoperation.

In the prior-art valve, the current I is increased to an upper limitI_(max) to which current can be passed under consideration of the coilresistance, power supply voltage, heat generation, etc., so that anoutput pressure P proportional to the current I can be obtained up tothe maximum pressure P_(max), as shown in FIG. 1(B).

In contrast with this, when the proportional pressure reduction valve 1of the present invention provided with such solenoid current-outputpressure characteristics as shown in FIG. 1(B) is used to control thepressure supplied to the clutch assembly 16, the solenoid current isdetermined below the predetermined relief current I_(r) and the outputpressure is determined below the relief pressure P_(r). This outputpressure is introduced into the clutch assembly 16 via the pipe 15, tourge the piston 35 in the leftward direction against the return spring36, so that the piston 35 engages two axially slidable clutch disks 33and 34 into half engagement conditions by precisely controlling theclutch pressure.

Thereafter, when the solenoid current is increased beyond this reliefcurrent I_(r) corresponding to a relief pressure, since the pressurerises sharply to the supply pressure P_(s), the piston 35 urges theclutch discs 33 and 34 into contact with the stopper 37 into tightclutch engagement conditions. Since these clutch discs 33 and 34 arefixed to the first and second rotary bodies 31 and 32, respectively bysplines in the rotary direction, a rotational force is transmitted fromthe first rotary body 31 to the second rotary body 32 or vice versa.

In the embodiment, since the clutch engagement pressure is equal to thesupply pressure, it is possible to obtain a sufficient tight engagementpressure between the two clutch disks 33 and 34 without sliding motionthereof by increasing the solenoid current to a value a little above thepredetermined relief current I_(r), thus saving the power consumption.

When the current is decreased from the tight clutch engagement conditiondown to the relief current I_(r), since the pressure within the feedbackchamber 23 is high, the spool 17 moves in the rightward direction, sothat the supply port 10 is closed; the drain port 11 is opened; thepressure within the output port 12 drops; and therefore the pressurewithin the feedback chamber 23 drops. When the pressure within thefeedback chamber 23 drops below the relief pressure P_(r), since theball 28 is urged to the throttle 25 by the return spring 27, the outputpressure decreases in proportion to the solenoid current below therelief current I_(r).

FIG. 1(C) shows a modification of the first embodiment, in which therelationship between the solenoid current and the output pressure ismodified.

When a pressure P_(b) determined by an expression as ##EQU1## is lowerthan the supply pressure P_(s), where R_(f) denotes the hydraulicresistance of the feedback path 21 and the throttle 22 and R_(r) denotesthe hydraulic resistance of the relief valve 29 and the throttle 25, itis possible to modify the current-output pressure characteristics asshown in FIG. 1(C). In FIG. 1(C), when the current reaches thepredetermined relief current I_(r), the output pressure changesmomentarily from the relief pressure P_(r) to the pressure P_(b), andthe output pressure increases up to the supply pressure in proportion toan increase in solenoid current when the current increases beyond thepredetermined relief current I_(r).

Further, in FIG. 1(A), since the relief valve 29 is disposed on theupper side of the proportional pressure reducing valve 1 when installedin position, it is possible to vent air within the feedback chamber 23to stabilize the pressure control operation of the valve.

FIG. 2(A) shows a second embodiment of the present invention, in whichthe same reference numerals have been retained for the similar partswhich have the same functions, without repeating the descriptionthereof.

In FIG. 2(A), the spool 39 is formed with a small diameter portion 40 atthe leftward end thereof. Further, the feedback chamber is divided intoa first feedback chamber 41 formed on the left end portion of a smalldiameter portion 40 and a second feedback chamber 43 formed by the smalldiameter portion 40, an inner circumferential wall near the supply port10, and the shoulder portion of the land portion 42 facing the supplyport 10. This second feedback chamber 43 communicates with the outputport 12 via a throttle 44 and a path 45 and the feedback path 21.Further, in the same way as in the first embodiment, the first feedbackchamber 41 communicates with the output port 12 via the throttle 22 andthe path 21 and the drain port 11 via the throttle 25 and the path 26.Further, the relief valve 29 is composed of the return spring 27, theball 28 and the throttle 25. The structural features and functionaleffects of this second embodiment other than those described above aresubstantially the same as with the first embodiment.

The operation of the second embodiment will be described hereinbelowwith reference to FIG. 2(B), in which the relationship between thesolenoid current and the output pressure is shown.

In FIG. 2(B), when the solenoid current I is equal to or lower than apredetermined relief current valve I_(r) corresponding to the reliefpressure P_(r), the operation is quite the same as in the firstembodiment.

Here, where A₁ denotes a pressure receiving area of the small diameterportion 40 of the spool 39 within the first feedback chamber 41; A₂denotes a pressure receiving area within the second feedback chamber 43,since the total pressure receiving area is (A₁ +A₂), an axial force F ofthe proportional solenoid section 2 required for balanced condition canbe expressed as

    F=P(A.sub.1 +A.sub.2)                                      (4)

where P denotes an output pressure or a feedback pressure.

Further, when the output pressure P is higher than the relief pressureP_(r), since the pressure within the first feedback chamber 41 is keptroughly at the relief pressure P_(r), the following expression can beobtained

    F=P.sub.r A.sub.1 +PA.sub.2                                (5)

Here, since an axial force F of the proportional solenoid section 2 isproportional to the current I passed through the coil 4, the gradient(the rate of change) of the output pressure P with respect to thecurrent I above the relief pressure P_(r) is larger than that below thesame relief pressure P_(r), so that the output pressure P reaches thesupply pressure P_(s) before the current I reaches the maximum currentI_(max), as depicted in FIG. 2B. Further, in FIG. 2(B), the dashed lineindicates the prior-art characteristics.

When the clutch pressure is controlled, the two clutch disks 33 and 34are engaged into a half clutch condition below the relief pressure P_(r)to gradually transmit a rotary force. Under these conditions, since afeedback pressure is applied to both the first and second feedbackchambers 41 and 43, the spool 39 can be held stably, without beingsubjected to external disturbance such as friction, hydraulic force,etc.

After a rotary force has been sufficiently transmitted, the currentpassed through the coil 4 is increased beyond the predetermined reliefcurrent to apply a sufficient pressure to the clutch assembly 16 toprevent clutch slipping operation. Under these conditions, since thefeedback pressure within the second feedback chamber 43 is applied tothe spool 39, although the pressure control precision is a littledeteriorated, this pressure is sufficiently high not to cause the clutchto be slided. In this second embodiment, it is possible to obtain highoutput pressure by a solenoid current smaller than that of the prior artrelief solenoid current.

FIG. 2(C) shows a modification of the second embodiment, in which theway of forming the feedback receiving area is different from that shownin FIG. 2(A).

In FIG. 2(C), the spool 47 is formed with a small diameter land portion18 facing the supply port 10 and a large diameter land portion 48 facingthe drain portion 48. In the same way as in the first embodiment, afirst feedback chamber 41 is formed at the left end of the spool 47.Further, an area A₂ to which the second feedback pressure within thesecond feedback chamber 43A is applied is obtained by subtracting apressure sensitive area of the land portion 18 on the groove side 20from that of the land portion 47 on the groove side 20. Further, theexpressions (4) and (5) explained in the second embodiment and thecurrent-output pressure characteristics shown in FIG. 2(B) can beapplied to these modifications as they are when the pressure receivingarea in the first feedback chamber 41 is denoted by A₁ and that in thesecond feedback chamber 43A is denoted by A₂.

FIG. 3(A) shows a third embodiment, in which the same reference numeralshave been retained for similar parts which have the same functions,without repeating the description thereof.

In FIG. 3(A), a piston 51 is slidably provided so as to face the spool17 within a feedback chamber 50 formed on the left end of the spool 17.A plunger is additionally provided on the left side of the piston 51,and the left end of the plunger 52 communicates with the supply port 10via the path 53. Therefore, this plunger 52 urges the piston 51 in therightward direction, and further the piston 51 is urged against thestopper 54. The space at which the piston 51 is in contact with theplunger 52 communicates with the drain port 11 via the path 55. Thepiston 51 is formed with a groove portion 56 to form an open/close valve57. The structural features and functional effects of this thirdembodiment other than those described above are substantially the sameas in the first embodiment.

The operation of this third embodiment will be described hereinbelow.With reference to FIG. 3(B), when current supplied to the coil 4 issmall and therefore the pressure at the output port 12 or the feedbackchamber 50 is low, since the supply pressure is applied to the left endof the plunger 52, the piston 51 is brought into contact with thestopper 54. Under these conditions, since the groove 56 does notcommunicate with the path 55, the feedback chamber 50 is closed on thepiston (51) side, so that the valve operates in the same way as in theprior-art valve.

When the pressure within the feedback chamber 50 or the output pressureincreases as

    P≧P.sub.A =P.sub.S A.sub.1 /A.sub.2                 (6)

where A₁ denotes a pressure receiving area of the plunger 52; A₂ denotesa pressure receiving area of the piston 51; and P_(S) denotes the supplypressure, the piston 51 moves in the leftward direction and the feedbackchamber 50 communicates with the drain port 11 via the path 55. Since anaxial force generated by the solenoid section 2 and applied to the spool17 increases into unbalanced condition, the spool 17 moves in theleftward direction; the supply port 10 is full opened; and the outputpressure P rises up to the supply pressure P_(S).

In the same way as in the first embodiment shown in FIG. 1(A), theoutput pressure increases to P_(A) when the current increases to I_(A),so that the pressure applied to the clutch assembly 16 is controlledprecisely under half clutch engagement condition between the two clutchdiscs 33 and 34 to gradually transmit a rotary force. Thereafter, thesupply pressure P_(S) is obtained by increasing the current to a valvemore than I_(A), so that a sufficient engagement power can be obtainedwithout producing clutch sliding operation.

Further, when current a little higher than I_(A) is passed through thesolenoid, since a high supply pressure P_(S) can be applied to theclutch assembly 16, it is possible to economize power consumption ascompared with the prior-art valve.

Further, as apparent from the expression (6), the output pressure P_(A)changes in dependence upon the supply pressure P_(S) in accordance witha proportional constant within a proportional control range. Therefore,it is possible to adjust the pressure valve P_(A) ' or P_(A) '' inrelation to the supply pressure P_(S) ' or P_(S) '" as shown in FIG.3(C).

Further, in this third embodiment, when it is not preferable to changethe hydraulic pressure from P_(A) to P_(S) (supply pressure) abruptly,the change in pressure can be reduced by providing an additionalthrottle within the path 55. Further, when the piston 51 is formed witha plurality of groove portions (three or more), since air within thefeedback chamber 50 can always be vented, it is possible to preventunstable operation due to air floated in the feedback chamber 50.

A fourth embodiment of the present invention will be described withreference to FIG. 4(A), in which the same reference numerals have beenretained for the similar parts which have the same functions, withoutrepeating the description thereof.

In FIG. 4(A), the spool 58 is formed with a land portion 18 facing thesupply port 10 and a land portion 59 facing the drain port 11. Thediameter of the land portion 59 is larger than that of the land portion18. The second feedback pressure is applied to an area obtained bysubtracting the pressure receiving area of the land portion 18 on thegroove side 60 from that of the land portion 58 on the groove side 60.

A first feedback chamber 61 is formed on the left end of the spool 58. Asecond spool 62 is disposed within the first feedback chamber 61coaxially with the spool 58. A spring 63 is disposed on the left end ofthis second spool 62 to urge the second spool 62 in the rightwarddirection into contact with the stopper 64. The left end of the secondspool 62 communicates with the drain port 11 via the path 65. Further,the second spool 62 is formed with two land portions 66 and 67, and agroove portion 68 formed between the two land portions. A path 69 isformed passing through the land portion 67 and the groove portion 68.

Further, a path 70 communicating with the path 65 is formed so as toface the land portion 66 at the position where the second spool 62 isbrought into contact with the stopper 64. Similarly, a feedback path 71communicating with the output port 12 is formed so as to face the landportion 67 and the groove portion 68 at the same position.

As described above, the second spool 62, the spring 63, the path 70, andthe feedback path 71 constitute a second pressure reducing valve 72 toreduce the pressure within the first feedback chamber 61. The structuralfeatures and functional effects of this third embodiment other thanthose described above are the same as in the first embodiment.

The operation of the fourth embodiment will be described hereinbelow.

With reference to FIG. 4(B), when the pressure within the first feedbackchamber 61 is equal to or lower than a preset relief pressure P_(r)determined by an elastic force of a spring 63 which urges the secondspool 62, since the second spool 62 is moved in the rightward directioninto contact with the stopper 64, the first feedback chamber 61communicates with the feedback path 71 via the path 69 formed in thesecond spool 62 (the path 69 does not communicate with the path 70).Therefore, the valve output pressure increases in proportion to thesolenoid current, as in the prior art.

When the pressure in the first feedback chamber 61 increases more than apredetermined relief pressure P_(r) of the second pressure reducingvalve 72, the second spool 62 moves in the leftward direction, so thatthe feedback path 71 is closed and the path 70 is opened so that thefirst feedback chamber 61 communicates with the drain port 11 via thepaths 69 and 70.

On the other hand, when the pressure within the first feedback chamber61 crops below the relief pressure P_(r) of the second pressure reducingvalve 72, since the second spool 62 moves in the rightward direction,the pressure within the first feedback chamber 61 is maintained at thepreset relief pressure P_(r) as it is.

As shown in FIG. 4(B), although the gradient of the output pressure withrespect to the solenoid current I passed through the coil 4 is smallwithin the range where I is I_(r) or lower, this gradient increaseswithin the range where I is more than I_(r). The pressure rises up tothe supply pressure P_(S) at the current lower than the maximum currentI_(max).

The pressure in the clutch assembly is controlled precisely under thepressure equal to or lower than P_(r) in the same way as in the priorart, in such a way that the two clutch discs 33 and 34 are engaged underhalf clutch conditions to gradually transmit a rotary force between thetwo discs. In this embodiment, since the first and second feedbackpressures are effective, the first spool 58 is held stably without beingsubjected to external disturbance such as friction, hydraulic pressure,etc.

Further, after a sufficient rotary force has been transmitted, thecurrent passed through the coil 4 is increased beyond I_(r), so that ahigh pressure is applied to the clutch assembly 16 to prevent clutchslipping operation. Under these conditions, since only the secondfeedback pressure is operative, although the clutch control precision islowered, there exists no problem when a hydraulic pressure high enoughto prevent clutch sliding operation is obtained. Further, since thesupply pressure P_(S) can be obtained by a current lower than I_(max),it is possible to reduce the current consumption.

Further, in the above fourth embodiment, it is also possible to make thediameter of the land portion 18 equal to that of the land portion 59 ofthe first spool 58. In this case, since no second feedback pressureproduced due to a difference in diameter between the two is produced andtherefore only the first feedback pressure produced in the firstfeedback chamber 61 is operative, when the solenoid current increasesbeyond I_(r) corresponding to the preset pressure P_(r) determined bythe second pressure reducing valve 72, the output pressure increasesmomentarily from P_(r) to P_(S), so that it is impossible to obtain anintermediate pressure.

A fifth embodiment of the present invention will be describedhereinbelow.

As already described, although the clutch transmission power delay timeis determined by delay of spool motion, pressure loss of theproportional pressure reducing valve, pressure loss of pipes, etc.,since viscosity of the working oil increases at low temperature, theabove delay time further increases. In practice, when the working oilviscosity increases at low temperature, since the hydraulic resistanceat the throttle within the feedback path increases and the oil is closedwithin the feedback chamber, the spool cannot move smoothly. Inaddition, since the pressure loss in the pipe from the output port tothe clutch assembly increases with increasing oil viscosity, thepressure applied to the clutch piston drops and therefore the pistonmovement becomes slow.

The viscosity of working oil increases exponentially with decreasing oiltemperature. In the case of torque converter oil, the viscosityincreases abruptly from -10° to -20° C. and changes to a thick maltsyrup state below -30° to -40° C. Therefore, the clutch assembly willnot operate smoothly when current is passed through the coil of theproportional solenoid section. Therefore, there exists a problem in thatthe delay time of the clutch response is as long as several seconds toseveral minutes, thus degrading the reliability of the valve.

To overcome this problem, there has been proposed a pressure reducingvalve in which the damping throttle is adjusted according to oiltemperature, as disclosed in Japanese Published Unexamined Utility ModelAppli. No. 61-139369. However, even if only the damping effect of thespool valve is reduced, since the pressure loss of the pipe is stilllarge, the response speed of the valve cannot be sufficiently improved.

This fifth embodiment provides a proportional pressure reducing valvewhich can improve the response speed of the feedback control valve atlow temperature below -20° C.

In FIG. 5A, a spring 74 made of shape memory alloy is used instead ofthe spring 27 of the relief valve 29 shown in the first embodiment. Thatis, an open/close valve 75 is composed of this spring 74, the throttle25 and the ball 28, which is closed at temperature above atransformation point of the shape memory alloy spring 74. The structuralfeatures and functional effects of this embodiment other than thosedescribed above is the same as in the first embodiment.

In FIG. 5A, when the hydraulic oil pressure rises beyond thetransformation point of the spring 74 (e.g. the normal temperature), thespring 74 extends to close the throttle 25, so that the open/close valve75 is closed. Therefore, the proportional pressure reducing valveoperates in the same way as in the prior-art valve as shown by a solidline A in FIG. 5B.

In contrast with this, when the hydraulic pressure temperature dropsbelow the transformation point of the spring 74, the spring 74contracts, so that the pressure within the feedback chamber 23 isrelieved to the drain port 11. Under these conditions, the pressurewithin the feedback chamber 23 is determined at a ratio in pressure ofthe throttle 22 of the feedback path 21 to the throttle of theopen/close valve 75. Therefore, when the solenoid current the same aspassed at high temperature is passed through the coil 4, the outputpressure at the output port 12 rises as shown by a solid line B in FIG.5B, which indicates the output pressure higher than that A obtained athigh temperature.

As long as the open/close valve 75 is closed at the normal temperature,the clutch pressure is controlled as in the prior-art valve. On theother hand, when the open/close valve 75 is opened at low temperaturebelow the transformation temperature, since the hydraulic resistance ofthe throttle 22 of the feedback path 21 increases, the working oildisplaced by the movement of the spool 17 is discharged through thethrottle 25. Therefore, it is possible to smoothly move the spool 17 toquickly open the supply port 10.

Further, when current the same as at high temperature is passed throughthe coil 4, since the pressure within the feedback chamber 23 is dividedinto two via the throttles 22 and 25, this pressure becomes low ascompared with at high temperature. Therefore, since the spool 17 movesin the leftward direction, the opening rate of the supply port 10becomes larger than at high temperature and therefore the pressure lossof the pressure reducing valve decreases. At the same time, since thepressure within the output port 12 at low temperature is higher than athigh temperature, even if the pressure loss in the pipe 15 is large, itis possible to supply a sufficient oil pressure to the piston 35 of theclutch.

Further, the working oil warmed at the hydraulic pressure source 13 iscirculated to the tank 14 by way of the supply port 10, the feedbackpath 21, the feedback chamber 23, the path 26 and the drain port 11, thetemperature within the pressure reducing valve rises. Therefore, it ispossible to markedly improve the response speed at low temperature dueto combination of the above-mentioned various effects.

As depicted in FIG. 5B, a higher pressure can be generated by the samecurrent at low temperature as compared with at high temperature.Therefore, a pressure higher than the pressure under which the twoclutch discs 33 and 34 are kept under the half clutch engagementconditions is applied to the piston 35, so that there exists a tendencythat the transmission shock is increased. However, at low temperaturesince the movement speed of the piston is low and the viscosity of theworking oil between the clutch disks 33 and 34 is also high, thereexists a tendency that the transmission shock is reduced.

Here, it is preferable to set the transformation temperature of theshape memory alloy spring 74 at about -20° to -30° C. at which the oilviscosity is sufficiently high.

FIG. 5C shows solenoid current-output pressure characteristics obtainedwhen an elastic force of the spring 74 is determined relatively small.In this case, since the open/close valve 75 opens when the pressurewithin the feedback chamber 23 exceeds this spring force at high (thenormal) temperature, the output pressure at high temperature as shown bya solid line A' rises momentarily to an output pressure obtained at thelow temperature as shown by a solid line B' in FIG. 5C.

FIG. 5D shows a modification of the fifth embodiment, in which a shapememory alloy spring 74 is used to construct the open/close valve 75,instead of the spring 27, in the second embodiment shown in FIG. 2A. Thestructural features and functional effects of this embodiment other thanthose described above are substantially the same as with the secondembodiment previously described.

In the fifth embodiment shown in FIG. 5A, the gradient of the solenoidcurrent-output pressure characteristics at low temperature is determinedby a ratio in pressure of the throttle 22 to the throttle 25, thisgradient changes according to oil temperature. In this modification,however, it is possible to make constant of this gradient irrespectiveof oil temperature.

That is, when the open/close valve 75 is closed at high temperature, thefeedback pressure is generated within the first feedback chamber 41 andthe second feedback chamber 43 so as to adjust the pressure.

When the open/close valve 75 opens at low temperature, since thehydraulic resistance of the throttle 25 is determined sufficiently smallas compared with the resistance of the throttle 22, the pressure withinthe first feedback chamber 41 is roughly equal to the pressure at thedrain port 11, so that the feedback pressure is obtained only within thesecond feedback chamber 43. Therefore, the pressure at the output port12 is set to a point a little higher than at high temperature. Further,the gradient of the output pressure with respect to the solenoid currentis determined by the pressure receiving area of the spool 39 within thesecond feedback chamber 43.

Further, it is possible to obtain the same effect as described above byuse of bimetal or wax expansion, instead of the shape memory alloyspring, in order to form the open/close valve 75 actuated in response tooil temperature.

As described above, in the proportional pressure reducing valveincluding a spool housed in a valve housing in such a way that an axialforce of a proportional solenoid is applied to one end thereof and anoutput pressure is feedbacked to the other end thereof via a throttleformed in a feedback chamber so as to be balanced, since the feedbackchamber communicates with a tank via a control valve after output portpressure increases gradually to a relief pressure, it is possible toincrease the output pressure momentarily up to the supply pressure. Thatis, it is possible to improve the control precision, widen controlpressure range, and increase response speed, without increasing the sizeof the proportional solenoid and the current passed therethrough, whenapplied to the clutch assembly. The valve output pressure below therelief pressure is used to engage the clutch in a half clutch conditionand the valve output pressure the same as the supply pressure is used toengage the clutch in a tight clutch condition, after a sufficient torquehas been transmitted.

What is claimed is:
 1. A proportional pressure reducing valvecomprising:(a) a valve housing formed with a supply port, an outputport, a drain port, and a feedback chamber communicating with the outputport; (b) a proportional solenoid attached to said valve housing; (c) afirst spool having a first end connected to said proportional solenoidand a second end exposed to the feedback chamber, said first spoolformed with a groove for allowing the supply port to communicate withthe output port, when said first spool is driven by said proportionalsolenoid, in such a way that output pressure at the output portincreases in proportion to current passed through said proportionalsolenoid under balanced conditions of an axial force applied to thefirst end of said first spool by said proportional solenoid and afeedback force applied to the second end of said first spool within thefeedback chamber; and (d) a pressure reduction valve including a secondspool formed with a path, for relieving hydraulic pressure within thefeedback camber to the drain port, when output port pressure risesbeyond a predetermined relief pressure determined by a spring, tosharply increase the output port pressure up to the supply pressure inproportion to the current passed through the proportional solenoid.